The invention relates to a method for elimination of foam above the level of a liquid, and particularly above the liquid content of a container filled with, for example, milk. The invention further relates to an apparatus for carrying out said method.
In many industrial areas it is desirable to avoid foam formation above the level of a liquid or to destroy foam which has formed. This is the case in flotation, for example, or in the evaporation of liquids, where foam located above the liquid level interferes with transportation and evaporation. The present invention is concerned specifically with the elimination of foam forming above the liquid level of containers which are being filled with liquids such as milk or fruit juices and which are then to be closed. Such foam, formed either during and/or as a result of the filling operation, is troublesome especially when containers fabricated from plastics-coated paperboard are being filled which are then closed at the top by heat sealing or bonding. When there is foam between the surfaces to be adhered, a proper bond cannot be secured in the areas so wetted, and the container so closed is not adequately sealed. With heat sealing, it is possible to bond the surfaces to be sealed since the heat generated and the pressure exerted by the sealing jaws will evaporate and/or squeeze off the foam film present. However, the protein residue remaining on the surfaces to be sealed will impair the seal. A foam film is particularly deleterious in a heat-sealing operation which involves preheating (activating) of a thermoplastic coating forming the surfaces to be sealed, followed by a pressing together of these surfaces by means of cold pressure jaws to form the seal. The wet foam will cool the preheated sealing surfaces in places so that in the pressing operation which follows a dependable bond, and hence an effective closure, is not secured.
A further drawback is that as the container is closed, as by folding of the top of a folding container, the foam over the liquid level overflows and fouls both the outside of the container and the sealing implements.
Since for reasons of economy the containers are not sized substantially larger than as required for accommodation of the contents, it is necessary in the cooling of milk, for example, in filling plants to provide a separate means for elimination of the foam over the milk if the drawbacks described are to be avoided. This means comprises a short suction pipe adapted to be introduced into the container to be sealed, and a suction system whereby the foam over the liquid level is removed. Moreover, to assure a satisfactory closure, blast nozzles are provided for blowing the drops and residual foam from the surfaces to be sealed or bonded.
This known foam-elimination method is afflicted with several drawbacks. In the first place, the milk sucked up as foam, which represents a substantial amount, cannot be reused to fill containers but can be used only as livestock feed. This amounts to a loss of milk to be packaged. Moreover, the tendency to foam in being packaged varies from one liquid to another and in the case of milk depends on its condition (raw milk or homogenized milk) and on its fat content, on the temperature, and on the rate at which the containers are filled with it. In fact, foaming may differ from one container to the next. As a result, the extent to which the container is filled with milk varies with the amount of foam drawn off. Finally, in the packaging of sterile liquids, such as sterilized milk, the drawing off of foam may render it difficult to maintain sterile conditions. The suction pipe comes in contact with the foam and therefore must be kept clean and inspected frequently if contamination with microorganisms is to be prevented.